1. Field of the Invention
The present invention relates to an indexable insert (also known in the art as a throwaway tip) for turning use and to a cutting tool configured such that the indexable insert is clamped on a tool holder.
2. Description of the Related Art
Finished-surface roughness (Ry) associated with turning is expressed by Ry≅f2/8R, where R is a nose radius, and f is a feed. In principle, an arc assumed by a nose of an indexable insert (the nose radius is usually 0.4 mm to 3.2 mm) is transferred intact to a cut surface, thereby determining finished-surface roughness. Accordingly, for a given feed, the greater the nose radius, the greater the improvement in surface roughness. However, since the width of a portion of a nose cutting-edge in contact with a workpiece increases with the nose radius, an increase in the nose radius can disadvantageously result in chattering. Therefore, an important measure is to determine the nose radius while a balance between improvement in surface roughness and prevention of chattering is achieved.
Turning a workpiece of cast iron involves, in addition to the above problems, the following phenomenon: since cast iron exhibits carbon segregation stemming from its constitution, turning results in digging-up the carbon segregated portions that are cut. Thus, even in finishing in which the depth of cut is small, an open bore, plucking, or the like arises on the finished surface, resulting in a failure to obtain a sufficiently improved finished-surface roughness. In order to improve the roughness of a machined surface of such a workpiece, known indexable inserts are configured such that a minor cutting edge (also called a xe2x80x9csweeper cutting-edgexe2x80x9d or a xe2x80x9cwiper cutting-edgexe2x80x9d) is formed between an arcuate cutting edge having a nose radius and a linear cutting edge (a side portion of a tip). The minor cutting edge assumes the form of a straight line substantially in parallel with the cutting feed direction (in cylindrical turning, the direction of the axis of rotation of a workpiece) or the form of an arc (curved line) having a radius different from the nose radius (See Patent Documents 1 to 3, identified below). An indexable insert having such a minor cutting edge is used to improve the shape transferred to a finished surface, to thereby improve theoretical surface roughness. An indexable insert having such a minor cutting edge is expected to improve surface roughness through the following mechanism: the minor cutting edge is engaged in cutting and burnishing to thereby plastically smooth a rugged machined surface.
In the indexable insert disclosed in Patent Document 1, a linear minor cutting edge having a length of 0.02 mm to 0.2 mm is provided between a nose portion and a linear cutting edge. In the indexable insert disclosed in Patent Document 2, an arcuate minor cutting edge is provided between an arcuate nose portion having a nose radius R and a linear cutting edge so as to satisfy the relation xe2x80x9c(nose radius) less than (radius of arcuate minor cutting edge).xe2x80x9d In the indexable insert disclosed in Patent Document 3, two arcuate cutting edges having radii r2 and r3, respectively, are provided between an arcuate nose portion having a nose radius (rxcex5) and a linear cutting edge as to satisfy the relation xe2x80x9cr2 less than rxcex5 less than r3.xe2x80x9d
[Patent Document 1]
Japanese Utility Model Registration No. 2556520
[Patent Document 2]
Japanese Kohyo (PCT) Patent Publication No. 8-507976
[Patent Document 3]
Japanese Kohyo (PCT) Patent Publication No. 10-500363
3. Problems to be Solved by the Invention
The results of various cutting tests conducted by the prevent inventors have revealed that use of the indexable inserts described in the above Patent Documents 1 to 3 can cause serious problems as described below. In the case of the indexable insert described in Patent Document 1, the length of the linear minor cutting edge (linear wiper cutting-edge) is as small (short) as 0.2 mm or less, which is too short to effect burnishing in turning a workpiece of cast iron as well as too short to be applied to a high-speed feed. Additionally, the linear minor cutting edge is set such that, when the cutting tool is fed (caused to travel) longitudinally in cylindrical turning, the minor cutting edge angle becomes 1 degree (1 degree in relation to the axis of rotation (hereinafter also referred to as the xe2x80x9caxisxe2x80x9d) of a workpiece) as measured on the rear side in relation to the direction of the feed.
However, the actual practice of cutting involves an error associated with clamping of the indexable insert in a tip pocket of a holder or an error associated with clamping of the holder on a tool rest. These errors are usually about xc2x11 degree. In some cases, such an error causes the indexable insert described in Patent Document 1 to assume a condition such that the linear minor cutting edge and the axis become parallel (form an angle of 0 degree) with each other. In such a case, cutting resistance increases, resulting in the occurrence of chattering. In the case where such a error causes the linear minor cutting edge to form an angle of 2 degrees with the axis, the cutting edge angle to a work surface increases, and thus machined-surface roughness (maximum roughness Ry) increases, resulting in impairment in finished-surface condition. That is, because of an unavoidable error in relation to the minor cutting edge angle, the indexable insert described in Patent Document 1 fails to consistently provide highly improved finished-surface roughness.
The indexable insert described in Patent Document 2 is designed to perform cutting such that the lowest point (a point at which a cutting edge is tangent to a line parallel with the axis of rotation) is located on the arcuate minor cutting edge, which assumes the form of a single arc having a radius greater than the nose radius, and thus can basically provide highly improved finished-surface roughness. However, since only the linear cutting edge, which is a portion of a side of the tip, is provided behind the arcuate minor cutting edge, the effect of burnishing a machined surface is not expected. For this reason, in the case of a workpiece of cast iron, a glossy finished surface cannot be obtained. In the case of turning by use of the indexable insert, in order to obtain a glossy finished surface, polishing must be performed after turning, disadvantageously resulting in an increase in machining man-hours.
In the case of the indexable insert described in Patent Document 3, since the nose radius (rxcex5) and the radius r2 of an arc following the nose have the relation xe2x80x9cr2 less than rxcex5,xe2x80x9d cutting by the cutting edge having the radius r2 impairs surface roughness. Thus, in order to improve surface roughness, cutting is performed such that the arcuate cutting edge having the radius r3, which is connected to the arcuate cutting edge having the radius r2 (r2 less than rxcex5 less than r3) on the side opposite the nose (on the side toward the linear cutting edge), includes the lowest point. However, in this case, for a reason similar to that in the case of the indexable insert described in Patent Document 2; i.e., since only the linear cutting edge is provided behind the arcuate cutting edge having the radius r3, the effect of burnishing a machined surface is not expected, thus resulting in a problem similar to that in the case of the indexable insert described in Patent Document 2.
In view of the above-described problems of conventional indexable inserts, an indexable insert of the present invention has been devised on the basis of the results of extensive cutting tests. That is, an object of the present invention is to provide an indexable insert which, in application to turning of a cast iron product, does not cause chattering and can provide a glossy, highly improved, finished surface.
The above object has been achieved by providing the following embodiments and aspects of the invention:
(1) An indexable insert adapted for turning, which comprises a nose cutting-edge having a nose radius R, a linear cutting edge, and a minor cutting edge formed between the nose cutting-edge and the linear cutting edge, wherein
the minor cutting edge comprises a first minor cutting edge having a radius Ra and a second minor cutting edge having a radius Rb, the first and second minor cutting edges successively extending from a nose tip (the center of the nose cutting-edge) to the linear cutting edge;
the nose cutting-edge and the first minor cutting edge are connected such that, at a connection point between an arc assumed by the nose cutting-edge and an arc assumed by the first minor cutting edge, a smaller intersectional angle xcex8x of a tangent T12 and a tangent T21 to the respective arcs is 1xc2x0 or less;
the first minor cutting edge and the second minor cutting edge are connected such that, at a connection point between the arc assumed by the first minor cutting edge and an arc assumed by the second minor cutting edge, a smaller intersectional angle xcex8y of the tangent T21 and a tangent T22 to the respective arcs is 1xc2x0 or less; and
the radius Ra of the first minor cutting edge, the radius Rb of the second minor cutting edge, and the nose radius R are set to the dimensional relationships xe2x80x9cRaxe2x89xa7R, Ra falls within a range of 3 mm to 20 mm, and Rbxe2x89xa71.5 Ra.xe2x80x9d
Preferably, both of the angles xcex8x and xcex8y are 1xc2x0 or less, since a sharp increase in the value of surface roughness can be suppressed.
(2) An indexable insert according to (1), characterized in that the connection point between the arc assumed by the nose cutting-edge and the arc assumed by the first minor cutting edge projects outwardly in relation to the adjacent arcs, and the connection point between the arc assumed by the first minor cutting edge and the arc assumed by the second minor cutting edge projects outwardly in relation to the adjacent arcs.
Formation of the outwardly projecting connection points facilitates working in the course of manufacture and is thus preferred.
(3) An indexable insert according to (1) or (2), characterized in that the angle xcex8x and/or the angle xcex8y is 0xc2x0.
Setting the angle xcex8x and/or the angle xcex8y to 0xc2x0 enhances the effect of suppressing a sharp increase in the value of surface roughness and is thus preferred.
In cylindrical turning using the indexable insert of the present invention, a cutting tool is fed longitudinally while the first minor cutting edge, which has a radius Ra greater than the nose radius R, serves as an end cutting edge. In this case, since Ra greater than R, the first minor cutting edge having a radius Ra contributes to improvement in theoretical surface roughness. Furthermore, the second minor cutting edge subsequent to the first minor cutting edge and having the radius Rb greater than the radius Ra of the first minor cutting edge rubs a cut surface to thereby burnish the cut surface. Thus, even in the case of a workpiece of cast iron, a finished surface becomes glossy; i.e., exhibits highly improved finished-surface roughness. When Ra is less than 3 mm, the burnishing effect attained by the second minor cutting edge is impaired, resulting in a failure to obtain burnish. When Ra is in excess of 20 mm, the width of contact of the end cutting edge with a workpiece increases, causing chattering and thus impairing surface roughness. When the radius Rb of the second minor cutting edge is less than 1.5 Ra, the burnishing effect becomes insufficient. The greater the value of Rb, the greater the attained burnishing effect. Thus, in terms of practice of burnishing, the second minor cutting edge may assume the form of a straight line.
(4) An indexable insert according to any one of (1) to (3), characterized in that the central angle xcex81 of the arc assumed by the first minor cutting edge falls within a range of 2 degrees to 4 degrees. When the indexable insert is clamped on a holder or when the holder is clamped on a tool rest, a clamping-angle error is unavoidable. Since the clamping-angle error is up to 2 degrees, employing a central angle xcex81 of 2 degrees or more suppresses impairment in finished-surface roughness. Employing a central angle xcex81 of 4 degrees or less decreases the length of a cutting edge exploited for cutting; thus, at a certain clamping angle, the width of contact of the first minor cutting edge with a workpiece is decreased, so that chattering becomes unlikely to occur. Therefore, a central angle xcex81 of 2 degrees to 4 degrees is preferred.
(5) An indexable insert according to any one of (1) to (4), characterized in that the arc assumed by the second minor cutting edge has a chordal length of 0.2 mm or more. The second minor cutting edge, together with the first minor cutting edge, performs burnishing. Thus, when the chord of the second minor cutting edge has a length of 0.2 mm or more, the width of a cutting edge exploited for burnishing becomes sufficiently large, so that burnishing is improved. Increasing the chordal length of the arc assumed by the second minor cutting edge allows an increase in the cutting feed.
(6) An indexable insert according to any one of (1) to (4), characterized in that the second minor cutting edge assumes the form of a straight line in place of the arc (namely, an arc having an infinite radius). As mentioned previously, the greater the radius Rb of the second minor cutting edge, the greater the burnishing effect. In other words, in terms of burnishing, as in the case of (4), the second minor cutting edge may assume the form of a straight line.
(7) An indexable insert according to any one of (1) to (6), characterized in that an outwardly projecting arcuate cutting edge is provided at a connection point between the second minor cutting edge and the linear cutting edge. In the case of contour turning in which a diameter-reducing taper portion of a workpiece is machined subsequent to cylindrical turning, and the connection point between the second minor cutting edge and the linear cutting edge comes into contact with the workpiece, a cutting edge including a jag (pointed portion) present at the connection point performs turning. As a result, the taper portion becomes inferior to the cylindrical portion in finished-surface roughness. However, when the indexable insert according to the invention described in (5) is used, a cutting edge region including the arcuate cutting edge provided at the connection point performs cutting, thereby preventing such impairment in surface roughness.
(8) A cutting tool (turning tool) comprising an indexable insert according to any one of (1) to (7) and a tool holder, the indexable insert being clamped on the tool holder.
Effect of the Invention
The turning a workpiece of cast iron using the indexable insert of the present invention or a cutting tool employing the indexable insert can prevent chattering and can provide a sufficiently glossy finished surface.